Plating of smooth,semibright gold deposits

ABSTRACT

A PROCESS AND PRODUCT FORMED BY ELECTROLYSIS OF AN AQUEOUS ACID GOLD BATH HAVING SOLUBLE THALLIUM SALT THEREIN FOR UNIFORM, SMOOTH DEPOSITION OF PURE GOLD.

United States Patent 3,562,120 PLATING OF SMOOTH, SEMIBRIGHT GOLD DEPOSITS Robert Duva, Paramus, and Atkin Simonian, Fort Lee, NJ., assignors to Sel-Rex Corporation, Nutley, N.J., a corporation of New Jersey 1 No Drawing. Filed Sept. 7, 1966, Ser. No. 577,618 Int. Cl. C23b /28 US. Cl. 204-46 15 Claims ABSTRACT OF THE DISCLOSURE A process and product formed by electrolysis of an aqueous acid gold bath having soluble thallium salt therein for uniform, smooth deposition of pure gold.

This invention relates to an electrolyte and to a process for plating gold.

Among the objects of the invention is to provide a gold plating bath and process having improved distribution.

The invention can be advantageously employed in any gold plating process, but is especially suitable for the gold plating of small parts according to the barrel plating process, for example. Small products, because of the work required to rack each separate product, are more effectively plated by a barrel plating process. When such parts are irregularly shaped to have small appendages or reentrant angles, the distribution of the plating obtained by conventional plating baths is uneven and such products must be excessively plated in some parts to obtain the minimum thickness of plating required in the parts where the distribution is poor. Since gold is expensive, the cost of plating by baths with poor distribution is excessive.

This invention is based on the discovery that the addition of certain limited amounts of soluble thallium compounds to acid gold plating baths of the type having a pH between 3 and 6, improves the metal distribution of the deposit and simultaneously improves the smoothness, color and brightness of the deposit obtained.

The gold is added to the acid gold baths in the form of alkali gold cyanide, preferably potassium gold cyanide. The gold can be present in amounts of l to 75 g./l. calculated as gold.

The baths contain conducting and buffering compounds and are adjusted to a pH of 3 to 6 by selecting a partially neutralized compound of a weak acid as the conducting media or by adding acid to the solution 'which contains a substantially neutral compound of a relatively weak acid. The minimum amount of conducting and buffering compound should be about 5 g./l. The maximum amount of acid and compound is not critical as long as the bath is adjusted to the pH range of 3 to 6 and as long as the resultant solution is not supersaturated at the temperature to be employed for electroplating.

The addition agent for the present invention is especially useful for the production of pure gold deposits, but can also be employed with alloy deposits.

The addition of the thallium compound of the invention does not aifect the hardness of the deposit obtained, but this invention is eminently suited for combination with the process as described in copending application Ser. No. 482,903, now Pat. No. 3,367,853, filed Aug. 23, 1965 and having the same assignee as the present application. Said application discloses the process of adding an alum to acid gold plating baths such as described above and by such addition, it is possible to increase the hardness of the deposited gold without including any detectable alloying element in the deposit. By adding various amounts of the alum in addition to the thallium compound addi- 3,562,120 Patented Feb. 9, 1971 "ice tions, it is possible to obtain smooth, bright yellow deposits of 24 karat gold, having a Knoop hardness, with a 25 gram load, within a range of to 145, for example. The alum is preferably added in the proportion of from 1 g./l. to 100 g./l.

The thallium compound is added in an amount to provide about 1 mg. to 25 g. of thallium, calculated as the metal, in the bath. Suitable thallium compounds are the thallous and thallic sulfates, thallium alum, thallium chloride, nitrate, fiuosilicate, or other water soluble thallium compounds.

The improved bath is operated at current densities of 0.1 to 10 amp/din. and at temperatures of about 20 C. to 90 C.

By way of a summary, the bath comprises the following ingredients, within the range specified:

As in said prior application, the alum added may be defined as a compound obtained by crystallizing the sulfates of aluminum, chromium and gallium with the sul; fates of ammonium, cesium, potassium, rubidium, sodium and thallium and with water in which the molar ratio is 1:1:24 and where M is the monovalent metal and M' is the trivalent metal.

Other objects and advantages of the invention will become apparent from the detailed description of the following examples.

EXAMPLE I A bath was made up of the following constitutents:

Diammonium citrate55 g./l.

Potassium chrome alum21 g./l.

Potassium gold cyanide (calculated as g0ld)8 g./l. Thallium sulfate300 mg./l.

Citric acid to pH5.3

A multiplicity of small parts having a head of about 1 cm. diameter and several thin leads extending from the 7 head were introduced into a conventional barrel plating device and plated in this solution at a current density of about 0.1 amp./dm. and at F. Parts placed in this solution to an average of 0.000050" thickness are smooth, lemon yellow and semi-bright and pass prescribed tests as follows:

(a) Heat treating in air at 500 C. for 5 minutes without discoloration.

(b) Die-attach test with a gold-silicon eutectic melt.

In the latter test, the device is heated to 410 C. and a pellet of gold-silicon eutectic is placed on the surface of the gold electroplate. The eutectic alloys with the gold plate and forms, when satisfactory, a smooth adherent melt which is not foamy, frothy or discolored.

Beta-ray backscatter measurements show that the gold deposit is approximately the same thickness throughout the part.

On the other hand, the same parts plated by said bath, but without the thallium addition, have a dark matte surface and many of the parts do not pass tests (a) and (b) above. The test of gold thickness by beta-ray backscatter shows that substantially greater thicknesses of gold are plated on the attachments than on the headers.

3 EXAMPLE II A bath was made up with the following constituents:

Potassium phosphate-100 g./l.

Gold as potassium gold cyanide-8 g./l. Thallium as the sulfate-2V2 mg./l. Phosphoric acid to a pH4.3

A polished brass panel was electroplated in this solution at a current density of about 0.3 amps./dm. at 60 C. A deposit of about 3 microns was smooth lemon-yellow and bright-although not as bright as the original brass panel. A similar bath made without the addition of the thallium produced a deposit at the same thickness which had a dull, orange-brown appearance. Gold deposits to thicknesses of 30-50 microns were also produced from this thallium containing bath. These were lemon-yellow in color, rather than the orange-yellow appearance of gold.

The features and principles underlying the invention described above in connection with specific exemplifications will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims shall not be limited to any specific feature or details thereof.

We claim:

1. An improved aqueous electrolytic gold plating bath especially useful in barrel plating, comprising:

Gold (added as alkali gold cyanide)1-75 g./l.

Conducting and bufiering compounds selected from partially neutralized weak acids-5 to 400 g./l. (saturated).

Thallium (added as a water soluble compound)0.001-

Adjusted to pH-36.

2. The bath as claimed in claim 1 wherein said alum is added in proportions of 1 to 100 g./l.

3. The bath as claimed in claim 2 wherein the alum added is potassium chrome alum.

4. The bath as claimed in claim 1 wherein said gold is added as potassium gold cyanide.

5. The bath as claimed in claim 1 wherein the conducting and buifering compounds are mixtures of partially neutralized Weak, stable, organic acids.

6. The bath as claimed in claim 1 wherein the conducting and buffering compounds are alkali phosphates.

7. The bath as claimed in claim 1 wherein the conducting and buffering compounds are a mixture of partially neutralized weak, stable, organic acids and phosphates.

8. A process of plating gold especially useful for small parts of irregular shape, comprising electrolyzing at a cur- 4 rent density of about 0.1 to 10 amp./dm. and at a temperature of about 20 C. to C., an aqueous bath of the following components:

Gold (added as alkali gold cyanide)-l-75 g./l.

Conducting and buffering compounds selected from partially neutralized weak acids-5400 g./l. (saturation).

Thallium (added as a watersoluble compound)0.'001

Alum-O-lOO g./l.

Adjusted to pH3-6.

9. The process as claimed in claim 8 wherein said alum is added in proportions of 1 to g./l.

10. The process as claimed in claim 9 wherein said alum is added as potassium chrome alum.

11. The process as claimed in claim 8 wherein said gold is added as potassium gold cyanide.

12. A process as claimed in claim 8 wherein the conducting and bufiering compounds are mixtures of partially neutralized weak, stable, organic acids.

13. A process as claimed in claim 8 wherein the conducting and buifering compounds are alkali phosphates.

14. A process as claimed in claim 8 wherein the conducting and buffering compounds are a mixture of partially neutralized 'weak, stable, organic acids and phosphates.

15. An aqueous electrolyte for the electrodeposition of pure gold comprising 1 to 75 g./l. of gold (added as an alkali gold cyanide), 5 to 400 g./l. (saturation) of conducting and buffering compounds selected from partially neutralized weak acids, and 0.001 to 25 g./l. of thallium (added as a water soluble compound), the pH of the electrolyte being adjusted to the range of from about 3 to about 6.

References Cited UNITED STATES PATENTS 2,393,905 1/1946 Hensel 75-165 2,396,100 3/1946 Hensel 75-165 2,967,135 1/1961 Ostrow et al 20443 3,021,266 2/1962 Ostrow et al. 204-44 3,367,853 2/1968 Schumpelt 20446 OTHER REFERENCES Brenner, Abner: Electrodeposition of Alloys, volume II, p. 158 (1963).

GERALD L. KAPLAN, Primary Examiner U.S. Cl. X.R. 20443 

